Okay, so I've finally come up with an explanation for Twilight Town being the tutorial world that actually makes sense and falls in line with everything we've seen so far!
So, I believe the beginning of KH3 will start off with the scene where we see Riku and Mickey take their briefcases from Yen sid and head toward the Realm of Darkness to save Aqua. [More to support this theory is that this is the first scene chronologically that we've seen for KH3, it even takes place before Sora returns to Yen Sid's tower before visiting Olympus] As mickey has mentioned, it is very difficult to access or escape the Realm of Darkness. But here's where it gets interesting, when Sora escaped the Realm of Darkness the first time, Sora ended up in Castle Oblivion, which is located in the Realm Between. So it stands to reason that there is the possibility of accessing the Realm of Darkness by first traveling to the Realm Between. Mickey likely knows this, and so he takes Riku to a location that is directly in the center of the Realm Between, Twilight Town! Once there, I believe the player will play as Riku and search for a way into the Realm of Darkness. I believe this will serve as the tutorial world for the basic mechanics of the game (attacking, magic, inventory, etc.). Once Riku and Mickey find the entrance to the Realm of Darkness, a cutscene will play of them entering the Realm of Darkness and the cutscene will transition to the cutscene of Sora running up the stairs of Yen Sids tower. From here, the player will go to Olympus as Sora and begin the second phase of the tutorial, which will introduce the new Athletic Flow mechanics (as seen from the demo gameplay, which had tutorials that all primarily focused on Athletic Flow). After defeating the Rock Titan and progressing through Olympus, Sora will likely decide to return to Yen Sid's. I believe a cutscene will play of Sora leaving Olympus and then the cutscene will transition to a cutscene of Riku and Mickey in the Realm of Darkness. From here, we take control of Riku and traverse the Realm of Darkness until we finally meet up with Aqua in the Dark Margin. The player will likely have to fight Aqua as Riku (Im guessing Aqua will be fully shrouded in darkness so Mickey and Riku dont actually know who they are fighting until the cutscene plays of Aqua revealing herself, kind of similar to Sora in Ventus's armor from DDD). Once the cutscene plays of Aqua revealing herself, Aqua leaves the Dark Margin and Riku and Mickey soon follow. Riku's cutscene plays of him jamming his shattered keyblade into the ground. Riku and Mickey then head to Yen Sids tower where the cutscene plays of Mickey and Riku walking through the door, and Sora, Donald, and Goofy are standing before Yen Sid. From here Sora will get his KH3 attire and start on his journey to save Roxas after learning that Aqua has fallen to the darkness. He then starts his journey by heading to Twilight Town, where he receives his new gameboy!
So what do you think? ðŸ˜
NOTE: Its also possible that the keyblade we see in the trailer that aqua picks up is actually Mickey's keyblade. Aqua probably succeeds in shattering Riku's Keyblade (and injuring him, putting him out of the fight) and disarming Mickey, giving her the opportunity to restrain Mickey in darkness as we see in the trailer.
So, I believe the beginning of KH3 will start off with the scene where we see Riku and Mickey take their briefcases from Yen sid and head toward the Realm of Darkness to save Aqua. [More to support this theory is that this is the first scene chronologically that we've seen for KH3, it even takes place before Sora returns to Yen Sid's tower before visiting Olympus] As mickey has mentioned, it is very difficult to access or escape the Realm of Darkness. But here's where it gets interesting, when Sora escaped the Realm of Darkness the first time, Sora ended up in Castle Oblivion, which is located in the Realm Between. So it stands to reason that there is the possibility of accessing the Realm of Darkness by first traveling to the Realm Between. Mickey likely knows this, and so he takes Riku to a location that is directly in the center of the Realm Between, Twilight Town! Once there, I believe the player will play as Riku and search for a way into the Realm of Darkness. I believe this will serve as the tutorial world for the basic mechanics of the game (attacking, magic, inventory, etc.). Once Riku and Mickey find the entrance to the Realm of Darkness, a cutscene will play of them entering the Realm of Darkness and the cutscene will transition to the cutscene of Sora running up the stairs of Yen Sids tower. From here, the player will go to Olympus as Sora and begin the second phase of the tutorial, which will introduce the new Athletic Flow mechanics (as seen from the demo gameplay, which had tutorials that all primarily focused on Athletic Flow). After defeating the Rock Titan and progressing through Olympus, Sora will likely decide to return to Yen Sid's. I believe a cutscene will play of Sora leaving Olympus and then the cutscene will transition to a cutscene of Riku and Mickey in the Realm of Darkness. From here, we take control of Riku and traverse the Realm of Darkness until we finally meet up with Aqua in the Dark Margin. The player will likely have to fight Aqua as Riku (Im guessing Aqua will be fully shrouded in darkness so Mickey and Riku dont actually know who they are fighting until the cutscene plays of Aqua revealing herself, kind of similar to Sora in Ventus's armor from DDD). Once the cutscene plays of Aqua revealing herself, Aqua leaves the Dark Margin and Riku and Mickey soon follow. Riku's cutscene plays of him jamming his shattered keyblade into the ground. Riku and Mickey then head to Yen Sids tower where the cutscene plays of Mickey and Riku walking through the door, and Sora, Donald, and Goofy are standing before Yen Sid. From here Sora will get his KH3 attire and start on his journey to save Roxas after learning that Aqua has fallen to the darkness. He then starts his journey by heading to Twilight Town, where he receives his new gameboy!
So what do you think? ðŸ˜
NOTE: Its also possible that the keyblade we see in the trailer that aqua picks up is actually Mickey's keyblade. Aqua probably succeeds in shattering Riku's Keyblade (and injuring him, putting him out of the fight) and disarming Mickey, giving her the opportunity to restrain Mickey in darkness as we see in the trailer.